The invention discloses a method for monitoring the three-dimensional temperature and thermal stress of an ultra-supercritical steam turbine rotor in real time. The method comprises the steps that a calculation formula for the temperature and the thermal stress of a monitoring point of the steam turbine rotor is expressed in a Green function mode according to the characteristics of heating of an ultra-supercritical steam turbine in the running process and an equation for heat conduction in a three-dimensional object, the Green function is calculated in an off-line mode by using a finite element method, and after the actual structure shape of the steam turbine and the cooling steam working state of the steam turbine are accurately simulated and are processed through various precision improvement methods, the temperature and thermal stress distribution are rapidly and accurately calculated by using the Green function. The obtained calculation result is quite close to the finite element calculation result, ideal calculation accuracy is achieved, a model is simple, calculation is rapid, information of changes of the temperature and information of changes of the thermal stress at any positions in the ultra-supercritical steam turbine rotor can be accurately monitored in real time, the overstressing phenomenon is avoided, safety of the steam turbine in the running process is protected, starting, shutdown and variable-load running of the steam turbine are guided, and economy of the steam turbine in the running process is improved.